It is a difficult thing to talk to someone about Christianity when they hold onto other beliefs with great strength. This is even harder when the religious beliefs also have a cultural aspect to them. In such cases, asking them to turn to Christ is actually asking them to turn their backs on their culture, family and friends. In such cases rational arguments do not have much success.
However, there is still plenty that you can do.
Firstly you can pray. As a Christian we beleive in a God who has control over the hearts and minds of all people. This is what we mean by him being sovereign. So you can pray that he may lead your friend and turn his heart towards God and save him. You can also pray for yourself that you may live the kind of life and say the kind of things that would make Christianity appealing to them.
Secondly, talk about your own experience of God rather than trying to engage and challenge them. Its a “softly softly” approach that they cannot take offense to. Take every opportunity to talk openly about how you became christian and what it means to you. Try to make it natural rather than forced. After all God is now the centre of your life and it makes sense that you talk about it natuarally and openly.
Thirdly, live a godly life before them. A godly life is one that is characterised by loving other people, seeking their good before your own, being quick to forgive and slow to take offense and so on. Such a life is very appealng to those who live for themselves. It makes you stand out as being different and people notice that. More importantly it shows that you take your Christian belief seriously, that your have integrity and that you are not asking them to do something that you yourself are not willing to do.
When you do have a conversation with them, do not be afraid of what you are going to say or of not having an answer. You have God on your side and he will give you that right words to say (even though they might not feel like it at the time). If you try your hardest and seek to honour God in what you say thn you have done right. If you don’t know an answer then let them know that you will try to find out, then find it out and tell them.
Finally, be patient and faithful to God. It can sometimes take many conversations, many experiences and many people all coming togther over time to bring someone to God. You may have a small part to play or a big part and you may never know what effect it has had till many years later if at all. But God is in control and can use all these things, even the small things you do, to bring someone to him.
In the end its God’s responsibility and role to make people Christian. Our responsibility is to present the message of Christ to them as best we can and to pray for them.
I hope this helps and please come back and ask another question if you have one.